Sugar Transport (Translocation) in Plants

Active learning works for sugar transport because students often confuse phloem function with xylem transport and diffusion. Hands-on models and dissections let them see pressure gradients in action, turning abstract pressure-flow concepts into visible, measurable changes.

• Year 11
• Biology
• 4 activities
• 30 min–50 min

Learning Objectives

1. Explain the pressure-flow hypothesis, detailing the roles of osmosis and turgor pressure in phloem sap movement.
2. Compare and contrast source and sink tissues, identifying factors that determine their roles in sugar transport.
3. Analyze how environmental factors, such as drought or aphid infestation, disrupt translocation and predict the impact on plant health.
4. Model the pressure-flow mechanism using a physical setup to demonstrate sap movement from high to low pressure areas.

Concept Mapping

⏱ 45 min·Pairs

Model Building: Pressure-Flow Apparatus

Pairs construct a model using dialysis tubing filled with sucrose solution, tied at one end, and submerged partially in a water bath with pressure simulated by height differences. They measure flow rate by collecting droplets over time and compare to plant diagrams. Discuss how active loading mimics leaf cells.

Prepare & details

Explain the pressure-flow hypothesis for the movement of sugars through the phloem.

Facilitation Tip: During Model Building, circulate with a meter stick and stopwatch to help groups measure flow rates and connect pressure changes to observable movement in the tubing.

Setup: Tables with large paper, or wall space

Materials: Concept cards or sticky notes, Large paper, Markers, Example concept map

• Understand
• Analyze
• Create
• Self-Awareness
• Self-Management

Concept Mapping

⏱ 50 min·Small Groups

Dissection Lab: Stem Girdling

Small groups girdle stems of fast-growing plants like beans, then track leaf wilting and root starch levels with iodine tests after one week. Compare girdled and control plants, noting phloem blockage effects. Record observations in tables linking to pressure-flow disruption.

Prepare & details

Differentiate between 'source' and 'sink' tissues in the context of sugar transport within a plant.

Facilitation Tip: During Dissection Lab, remind students to keep the girdled stem submerged in water to prevent air embolisms that could obscure phloem blockages during the starch test.

Setup: Tables with large paper, or wall space

Materials: Concept cards or sticky notes, Large paper, Markers, Example concept map

• Understand
• Analyze
• Create
• Self-Awareness
• Self-Management

Concept Mapping

⏱ 30 min·Whole Class

Mapping Activity: Source-Sink Diagrams

Whole class annotates large plant diagrams identifying current sources and sinks, then redraws for fruiting versus dormant stages. Groups predict transport paths under stress like shading one leaf. Share and debate predictions using evidence from readings.

Prepare & details

Analyze how environmental factors or plant damage might disrupt phloem transport and affect plant growth.

Facilitation Tip: During Mapping Activity, provide printed outlines of plants so students can annotate source-sink roles and test predictions by shading specific leaves before revising their maps.

Setup: Tables with large paper, or wall space

Materials: Concept cards or sticky notes, Large paper, Markers, Example concept map

• Understand
• Analyze
• Create
• Self-Awareness
• Self-Management

Concept Mapping

⏱ 35 min·Individual

Inquiry Demo: Aphid Feeding Simulation

Individuals use syringes to extract 'sap' from model phloem tubes under microscopes, observing pressure drops. Extend to group discussion on real aphid damage via videos. Hypothesize yield losses and test with simple mass measurements.

Prepare & details

Explain the pressure-flow hypothesis for the movement of sugars through the phloem.

Facilitation Tip: During Inquiry Demo, ask students to predict feeding sites on a model aphid stylet before placing it on the stem, then compare their predictions to the actual feeding locations under magnification.

Setup: Tables with large paper, or wall space

Materials: Concept cards or sticky notes, Large paper, Markers, Example concept map

• Understand
• Analyze
• Create
• Self-Awareness
• Self-Management

Teaching This Topic

Teachers approach this topic by anchoring lessons in tactile models and direct observation, which counters the passive misconception that sugars move by diffusion alone. Avoid over-reliance on diagrams without concrete evidence, and instead use timers, rulers, and dissection tools to make pressure gradients tangible. Research shows students grasp bidirectional flow better when they manipulate phloem mimics and see immediate consequences of pressure changes.

What to Expect

Students will explain how pressure gradients drive sap movement and identify source-sink relationships. They will use evidence from their models and dissections to justify their reasoning and correct common misconceptions about phloem transport.

Watch Out for These Misconceptions

Common Misconception

During Model Building: Pressure-Flow Apparatus, watch for students assuming phloem transport is unidirectional like xylem.

What to Teach Instead

Use the pressure gauge and flow direction arrows on the tubing to show that pressure can push sap both up and down, and have students adjust the tubing height to demonstrate bidirectional flow under different pressure conditions.

Common Misconception

During Dissection Lab: Stem Girdling, watch for students believing girdling only affects water transport.

What to Teach Instead

After removing the girdle, have students perform an iodine starch test on root and leaf samples to show that phloem disruption starves sinks of sugars, not just water.

Common Misconception

During Mapping Activity: Source-Sink Diagrams, watch for students assuming all leaves are equally active sources.

What to Teach Instead

Provide shaded leaf cutouts and a lux meter so students can measure light exposure and revise their maps to show that shaded leaves may import sugars instead of exporting them.

Assessment Ideas

Quick Check

After Mapping Activity: Source-Sink Diagrams, collect student maps and ask them to add arrows showing sugar flow between two labeled sources and two labeled sinks, then provide feedback on their labeling and arrow accuracy.

Discussion Prompt

After Dissection Lab: Stem Girdling, pose the scenario and have students use their girdling results and pressure-flow hypothesis to explain what will happen to the leaves and roots, then facilitate a class consensus on the mechanism behind the observed effects.

Exit Ticket

After Inquiry Demo: Aphid Feeding Simulation, ask students to write one sentence explaining how osmosis contributes to sugar transport in phloem and one sentence describing a factor that could impede this transport, collected as they leave the lab.

Extensions & Scaffolding

• Challenge students to design a pressure-flow model using different tubing diameters and predict how each would affect sap movement speed.
• For students who struggle, provide pre-labeled source-sink diagrams with missing arrows for them to complete using their girdling lab results.
• Deeper exploration: Assign a case study of a fruit tree with uneven ripening and ask students to use source-sink mapping to explain the pattern and propose solutions.

Key Vocabulary

Translocation	The movement of sugars, primarily sucrose, from areas of production (source) to areas of storage or use (sink) within a plant via the phloem.
Phloem	The vascular tissue in plants responsible for transporting sugars produced during photosynthesis from the leaves to other parts of the plant where they are needed for growth or storage.
Sieve tube elements	The principal conducting cells of the phloem, arranged end to end to form sieve tubes, through which sap flows.
Source tissue	Plant tissues, typically mature leaves, that produce sugars through photosynthesis and export them to other parts of the plant.
Sink tissue	Plant tissues, such as roots, fruits, flowers, or developing leaves, that import sugars from source tissues for growth, storage, or metabolism.
Pressure-flow hypothesis	The theory explaining translocation, which states that bulk flow of phloem sap is driven by a pressure gradient established by the loading of sugars at the source and unloading at the sink.